However I also think that some of the bigger suppliers buy a lot at a time and store them in poor conditions for too long. The solvents can gradually evaporate, making them more brittle. Also sunlight is one of the few things that breaks down rubber. They need storing in dark cool slightly damp conditions ideally. In the olden days they were wound round with yellow celophane to help achieve this.
I'am not really sure if that is what is happening - just my suspicions.
I think your suspicions are almost certainly correct - and I'd forgotten about the old practice of wrapping tyres in cellophane (just like Lucozade) until you mentioned it. In the old days when tyres were sold by local bike shops, stock could be very slow to turn over, and packaging and storage are probably more geared towards the high turnover online suppliers these days. And smaller suppliers can suffer, as I suspect a lot of them buy up surplus stock from the big ones.
The tyre failures we've seen in this thread do look very similar to the way very old tyres fail (and very much like the tyres I discarded when I rebuilt an old bike of mine a couple of years ago that hadn't been used for 20 years). It's a similar kind of failure that happens when the rubber "dries out" and loses flexibility.
Storing spare tyres in a cool, dark and slightly damp place is something I learned from my grandfather many years ago, and I still do it now. I think it's particularly important if you stock up on tyres when you find them going cheap, as I do, as they're more likely to be old stock tyres to start with. I have a stock of tyres I got from SJS Cycles, who often have tyres in sizes that nobody else has (for example, my Marathon Greenguards and Gatorskins in 27 x 1 1/4), and I have an outhouse kind of place to store the spares, with no sunlight and definitely cool and damp. I've also bought tyres on offer and found they have an older tread pattern than current, which again suggests older stock.
I have to add that all the tyres I've bought from SJS have been excellent (and I expect they've been well stored for the time the company has had them), and it's reassuring that SJS has come up with good customer service in this case.
Oh, just add... It was "accepted knowledge" when I was in my teens that tyres left standing in the sun to get too hot had a greater chance of failing and egging.